It's probably the combination of a sparse frequency spectrum and
room reverb. The tone is at about 3 kHz with limited harmonic
structure, so there may be a recognisable ILD, but you can't tell elevation
or front from back. The beep is brief, so you can't use head movement
to disambiguate front and back. Then, the reverb will reduce the ILD and
make it change from point to point in the room. The precedence effect
should compensate for that to some extent, but the resulting perceived
azimuth is less stable for a tone than for a broadband signal. See, for
instance (fig 1.)....
http://symphony.arch.rpi.edu/~braasj/.inside/classes/AP2008/StudyMaterials3_Blauert08/blauertbraasch2005.pdf
If only it would produce a click train instead.
John.
>>> "Jennifer M. Groh" <jmgroh@xxxxxxxx> 25 June 2013 09:46 >>>
Dear List,
I am writing a book for a general audience on how the brain processes
spatial information ("Making Space"). The chapter on hearing covers many
topics in sound localization, but there is one that I'm currently still
quite puzzled about: why it is so hard to localize a smoke detector when
its battery starts to fail? Here is what I have considered so far:
- To my ear, the chirp sounds high frequency enough that ILD cues should
be reasonably large.
- At the same time, it seems to have a broad enough bandwidth, and in
any case it has onset-and-offset cues, that ITD cues should be usable.
- A possibility is that the chirp is too brief, and that limits dynamic
feedback, i.e. changes in ITD and ILD as the head turns during a sound.
However, in my laboratory we have obtained excellent sound
localization performance in head-restrained monkeys and human subjects
localizing sounds that are briefer than the reaction time to make an
orienting movement.
- An additional possibility is that we have too little experience with
such sounds to have assembled a mental template of the spectrum at the
source, so that spectral cues are of less use than is normally the case.
I'm leaning towards a combination of the last two factors, which
together would render the cone of confusion unresolved for these stimuli.
Thoughts?
Best wishes,
--Jennifer Groh
--
Jennifer M. Groh, Ph.D.
Professor
Department of Psychology and Neuroscience
Department of Neurobiology
Center for Cognitive Neuroscience
B203 LSRC, Box 90999
Durham, NC 27708
919-681-6536
www.duke.edu/~jmgroh